There is an increasing demand for high quality premium coke for the manufacture of large graphite electrodes for use in electric arc furnaces employed in the steel industry. The quality of premium coke used in graphite electrodes is often measured by its coefficient of thermal expansion (CTE) which may vary from as low as -5 to as high as +8 centimeters per centimeter per degrees centigrade times 10.sup. -7. Users of premium coke continuously seek graphite materials having lower CTE values, where the lower the CTE the higher the coke quality. Even a small change in CTE can have a substantial effect on large electrode properties. Another property which is of importance in characterizing the quality of graphite electrodes is density. The higher the density the better the electrode quality.
Premium coke is manufactured by delayed coking in which heavy hydrocarbon feedstocks are converted to coke and lighter hydrocarbon products. In the process the heavy hydrocarbon feedstock is heated rapidly to cracking temperatures and is fed continuously into a coke drum. The heated feed soaks in the drum and its contained heat which is sufficient to convert it to coke and cracked vapors. The cracked vapors are taken overhead and fractionated. The fractionator bottoms are recycled to the feed if desired. The coke accumulates in the drum until the drum is filled with coke, at which time the heated feed is diverted to another coke drum while the coke is removed from the filled drum. After removal, from the drum, the coke is calcined at elevated temperatures to remove volatile materials and to increase the carbon to hydrogen ratio of the coke.
It is desirable to operate the delayed coking process at low temperatures to enhance the development of the intermediate crystalline phase (mesophase) which results from the coking process. The more developed the mesophase prior to solidification of the coke the more crystalline is the final product, and in general, the lower the final product CTE. A major problem which is encountered when carrying out delayed coking at lower temperatures is the presence of unconverted feed or partially formed mesophase in the coke drum at the end of the coking process.
The feedstocks used for premium coke production typically produce between 20 and 45 weight percent coke. In general, about 50% or more of the feedstock in the liquid phase at coking conditions. The total vapor flow through the coke drum from the feed is significantly less than that produced by the same liquid volume rate of a material which is 100% vapor at coking conditions. A number of references discuss the use of a heat treating step wherein the delayed coking process is followed by contacting the coke with a non-coke forming material which is in the vapor state at the coking conditions employed. The prior art very clearly teaches that non-coking materials must be used. When this type of process is used a high vapor flow rate is required to maintain the coking temperature in the coke drums. As a result the unconverted feed and partially formed mesophase which are in the coke drum at the time of the switch from coking feed to non-coking vapor, is converted to foam. In turn, the foam is converted into a low density macroporous "fluff" coke at the end of the coking cycle. Fluff coke is very frangible and generates a large amount of fines when it is drilled out of the coke drum, during initial sizing and during calcination. The fine particles formed from fluff coke which "pass" through the calcination have a very low density and very little "needlelike" character. These characteristics create problems when the fluff coke particles are included in mixtures used for the manufacture of graphite electrodes because they significantly increase the pitch requirements. When insufficient pitch is provided, weak spots are created in the electrode by the fluff coke particles. The fluff coke also decreases the profitability of the premium coking operation by reducing the net production of coke. The low density fluff coke takes up much more volume in the coke drum per unit weight of coke.
It would be desirable to provide a delayed coking process which is carried out at a low temperature, and which utilizes a heat soak step, but at the same time, provides a premium coke product having a low CTE and substantially reduced in fluff coke content.